In general, a main function of a demodulator in a radio device that performs radio digital communication is to perform carrier reproduction or clock reproduction under the influence of phasing, etc. which may deteriorate channel quality. Meanwhile, with an increase in the operational frequency of the radio device and a reduction in the price thereof, influence of phase noise caused by an oscillator (local oscillator) incorporated in the radio device exerted on the characteristics of the entire device becomes greater. Under such a circumstance, the demodulator has fallen into a state of not being able to obtain sufficient characteristics under a condition that a degradation component caused by phase noise has been added. As described above, at the present day, the phase noise generated in a local oscillator can be taken as one of the problems that affect the quality of high-frequency digital communication.
For explaining a transmitter used for high-frequency digital communication, schematic configurations of a transmitter and a receiver are illustrated in FIG. 12. FIG. 12A illustrates a configuration of a transmitter, and FIG. 12B illustrates a configuration of a receiver. As illustrated in FIG. 12A, in a transmitter 40, a BB (Base Band) signal 46 is converted into an IF (Intermediate Frequency) signal 47 by an orthogonal modulator 41 and an IF local oscillator 42. Further, the IF signal 47 is converted into an RF signal 48 by an mixer 43 and an RF (Radio Frequency) local oscillator 44 and, finally, the RF signal 48 is output from an antenna 45. On the other hand, as illustrated in FIG. 12B, in a receiver 60, the signal output from the transmitter 40 is received by an antenna 66. Then, an RF signal 67 received by the antenna 66 is converted into an IF signal 68 by an mixer 64 and an RF local oscillator 65 and further converted into a BB signal 69 by an orthogonal demodulator 62 and an IF local oscillator 63.
In this case, four oscillators (RF local oscillators 44 and 64, IF local oscillators 42 and 63) in total are used in the transmitter 40 and receiver 60. The phase noises generated from the four local oscillators are uncorrelated with one another. In the configuration illustrated in FIG. 12, correction of the phase noise is performed at the same time when frequency error between the local oscillators on the transmitter side and receiver side is performed by a carrier reproduction PLL (Phase Locked Loop) 61 after conversion of the received RF signal 67 into the BB signal 69. However, even with such a method, it has been difficult for the reproduction PLL 61 to perform satisfactory correction when a strong phase noise component is present, resulting in degradation in carrier reproduction performance.
As a technique relating to this point, for example, Patent Literature 1 discloses a frequency conversion device in which an increase in frequency accuracy and reduction in the size and cost of the device are achieved. The frequency conversion device generates only some signals of low frequencies among signals of local oscillation frequencies by a phase locked loop according to a beacon signal of intermediate frequency generated by mixing a predetermined beacon signal with a signal of local oscillation frequency. By using the generated signals to perform frequency conversion, this technique aims to cancel frequency error or phase noise generated at the conversion time.
Further, for example, Patent Literature 2 discloses a demodulator including a digital signal generation unit, a timing reproduction unit, a carrier reproduction unit, and a phase noise correction unit. In the demodulator, the phase noise correction unit outputs a phase noise correction signal which has a value of increment in the case of plus increase of the phase difference and has a value obtained by multiplying increment by −1 in the case of minus increase of the phase difference. The carrier reproduction unit sets a gain of the phase difference of the digital signal after timing reproduction based on the phase noise correction signal and rotates a symbol in the phase noise suppressing direction by an oscillation signal oscillated in accordance with the gain to perform carrier reproduction. By adopting the above configuration, this technique aims to improve quality and reliability by efficiently performing correction control of phase noise with high precision.